in case anybody else runs into this problem:
yesterday my pi 3B would not boot after rpi-eeprom rpi-eeprom-images update.
I plugged the SSD into my pi4, saw nothing in the logs, finally noticed that the /boot partition was full. The SSD was a clone from an old image from 2016, so /boot was only 64m.
With gparted and this tutorial: https://learn.adafruit.com/resizing-ras ... partitions
I resized the partitions, /boot to 256m, copied the files from https://github.com/raspberrypi/firmware onto /boot
and the system came up again. Nftables did not work until I did rpi-update.
Now everything works again